SI units

kelvin (K)

The kelvin is the SI base unit for thermodynamic temperature

Accurate temperature measurement is widely important, from health to affecting the rate of chemical reactions.

We used to measure the temperature of an object by comparing it with the temperature of the triple point of water, which is exactly 273.16 K and is the temperature at which water can exist as a solid (ice), liquid and a gas (water vapour).  Unusually in the SI, we also define another unit of temperature, called the degree Celsius (°C). This is related to the kelvin by subtracting 273.15 from the numerical value of the temperature expressed in kelvin.

Comparison with a fixed temperature is not practical for accurate measurement of extremely hot or cold temperatures. We now define and measure temperature in terms of the energy of molecular motion. The relationship between degree Celsius and  kelvin is the same under different definitions.


The kelvin is defined by taking the fixed numerical value of the Boltzmann constant k to be 1.380 649 × 10−23 when expressed in the unit J K−1, which is equal to kg m2s−2 K−1, where the kilogram, metre and second are defined in terms of h, c and ∆ν.

This was a new definition in May 2019.

More about the redefinition in May 2019


Temperature is important to the operation of almost all industrial processes, from food preparation to the manufacture of microcircuits.

Many physical properties of things change with temperature, leading to many different ways to make thermometers. Once calibrated, thermometers use changes in the size of objects, changes in their electrical resistance and changes in colour to find out the temperature.

Did you know?

  • The coldest possible temperature (0 K or -273.15 °C) is called the absolute zero of temperature. At this temperature molecules would have – for all practical purposes – lost all their energy of motion
  •  Stainless steel is heated to over 1600 °C during manufacture

The science behind the unit

The accurate measurement of temperature is vital across a broad spectrum of human activities, including materials processing (e.g. making steel), manufacturing (e.g. parts only fit perfectly at a certain temperature), food (preparation, transport and storage), health, and of course scientific discovery. In fact, in almost every sector, temperature is one of the key parameters to be measured.

One difference between temperature and other physical properties, such as mass or length, is that it is intensive. If we consider two objects with the same mass and temperature, then their combined mass is the sum of the masses of the individual objects. However, their combined temperature will be unchanged. So, although it is fairly easy to imagine ways in which we can determine how much heavier one object is than a standard mass, it is not at all obvious how to determine how much hotter one thing is than another.

Temperature affects a wide variety of physical processes because all substances are composed of atoms, and fundamentally temperature is a measure of the average energy of the motion of the atoms within an object. The kelvin is defined in terms of this microscopic motion and is based on a fundamental constant known as the 'Boltzmann constant' that measures how much energy of motion corresponds to one kelvin.

This definition has that advantage that any fixed point can be used as a standard temperature, and any appropriate method for temperature measurement could be used. This allows for the possibility of improved uncertainty of temperature measurement at extremely high and extremely low temperatures.

NPL has a world leading team of researchers working in this area. We help organisations to understand the impact of reliable temperature and humidity measurement on their processes, solve their challenging measurement problems and enable innovative improvements in measurement techniques.

Temperature and humidity